Toll telephone system and method for maintaining or initiating loop current after resuming on-hook condition

ABSTRACT

A toll telephone connected to a coin trunk including tip and ring conductors in which loop current flows from the central office and through the telephone responsive to an off-hook condition at the telephone includes electronic circuitry for maintaining the loop current flow through the telephone for a predetermined period of time after the hook switch is opened, in order to accomplish certain telephone functions. Those functions include operation of an escrow relay, prevention of hook switch fraud, prevention of central office call termination until the call is legitimately completed and completion of certain call accounting functions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the telephone art, and particularly totoll telephone systems and methods.

2. Description of the Prior Art

Prior art telephone systems utilize a telephone instrument whichreceives battery and loop current inputs from a central office. Aconventional coin telephone is typically provided with an escrow relaythat holds the coins in escrow until completion of the telephone call.After the telephone resumes the on-hook condition, the central officebattery voltage is manipulated to operate the escrow relay and collectthe escrowed coins into the coin box of the telephone.

SUMMARY OF THE INVENTION

The present invention has as its purpose the provision for a tolltelephone system and method in which the telephone is not powered fromany external source, and is particularly adapted for a telephone systemand method, described in greater detail below, in which the telephoneelectronic circuitry is powered entirely from the direct currentcomponent of the loop current. In those circumstances, there is a needfor maintaining loop current for a predetermined period of time afterthe telephone resumes the on-hook condition, in order to accomplishcertain desired functions.

To this end, the telephone is provided with a hook switch for closing acircuit path between the tip and ring conductors forming a coin trunkwhich is coupled to a central office, and through which loop currentflows responsive an off-hook condition being sensed at the centraloffice.

In accordance with the present invention, the telephone is provided withmeans for maintaining loop current flow through the telephone for apredetermined period of time after the hook switch is opened. This meansfor maintaining the loop current flow is for a period of time sufficientto operate the escrow control relays, for prevention of any attempt athook switch fraud, for preventing central office hang-up until the callis legitimately terminated, and further for a period of time sufficientto permit the telephone to complete all call accounting functions.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B together are schematic illustrations, partially in blockdiagram form, of the electronic circuitry for a telephone system inaccordance with the present invention.

FIG. 2 illustrates pin connections of a microprocessor circuit andassociated electronics used with the telephone system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to FIGS. 1A, B and 2, in which various reference numeralsinclude letter prefixes to designate certain characteristics of theelectronic circuitry. For example, the prefix "R" refers to a resistor;the prefix "C" refers to a capacitor; the prefix "Q" refers to atransistor; the prefix "K" refers to a relay; the prefix "CR" refers toa diode; the prefix "U" refers to an integrated circuit device and theprefix "I" refers to an inverter. These reference letters areconventionally used in the electronic arts.

Other conventional designations are used in accordance with thetelephony art. For example, the terms "tip conductor" and "ringconductor" refer to those circuit conductors which are coupled to acentral office for providing a conventional loop current through thetelephone unit to facilitate various telephone functions. The term"ground" is intended to refer to some common electric reference point.

It will be understood by those skilled in the art that the telephoneshown in the drawing is associated with a central office having tip andring conductors which are respectively attached to the "tip" lead and"ring" leads of the schematic shown in FIG. 1A. Additionally, a "ground"GND connection is provided. It is important to note that there are twodifferent logic grounds shown in the drawing, including a non-switchedlogic ground which is always applied for either on or off-hookconditions, and a switched ground reference which is applied only whenthe telephone is in the off-hook condition; a legend for these two logicgrounds is shown in the upper left hand corner of FIG. 1A. The switchedground is at a 5-volt potential in the on-hook condition. For purposesof discussion, all of the relays are shown in the deenergized state. Aswill be hereinafter described, the telephone shown in the drawingrequires no connection to an external power source; rather, all powerrequired to energize all of the electrical and electronic circuitry ofthe telephone is derived from the loop current flowing through the tipand ring conductors from the central office and through the telephoneduring an off-hook condition. While a 6-volt battery B1 is shown in thedrawing, this battery only provides a backup and remains in a chargingstate during the off-hook condition. During on-hook conditions, thetelephone shuts down all power consuming circuits and completelydisconnects the battery B1 from the circuit when the phone is on-hook.

The particular features of the electronic circuity of the telephone inaccordance with the present invention and shown in FIGS. 1 and 2 willnow be described with reference to the various operational conditions ofthe telephone.

b 1. Idle State (On-hook Condition)

During the on-hook condition, transistors Q4 and Q5 are in the offstate; therefore, only a very small amount of leakage current flowsbetween the tip and ring conductors from the central office and throughthe telephone (i.e. on the order of 5 microamperes or less). Thisleakage path is through resistor R47. The central office requires 20milliamperes or more of loop current to qualify an off-hook condition.The leakage current path just described further includes the following:negative battery from central office to the ring conductor, through thenormal current detector U10, through the bridge CR11 to resistor R47 andthence connected to the negative terminal of CR11, which in turn iscoupled to the unswitched ground through resistor R47. The leakagecurrent continues to flow up through the negative side of capacitor C4to the positive terminal, which is the 5-volt buss and thence throughthe speech hybrid circuit U11 to the positive terminal of CR11, to thetip conductor, back to earth ground connection in the central officewhich is positive central office battery. An important aspect of thepresent invention is the utilization of a relatively large capacitor forelement C4; typically, capacitor C4 has a very high capacitance, on theorder of 0.22 farad and is therefore referred to as the "supercapacitor" of the circuit.

As a result of the leakage current conduction described above, capacitorC4 is maintained in a charged state, providing operating voltage tointegrated circuits U12 and U103. Both of these integrated circuits areCMOS devices, and therefore require operating currents well below 5microamperes.

During the on-hook condition, transistors Q2 and Q12 are off andtherefore a logic "zero" is present on the emitter of Q12 and the gateof integrated circuit U12 as a result of which the field effecttransistor Q11 is in the off state. Therefore, the switch ground isopen. Since the switch ground is open to the microprocessor U4 and theEPROM U3 and E2ROM U13 and U14, then a majority of the circuits in thetelephone are in a powered down condition. Transistor Q7 is also in theoff condition and therefore Q6 is off which disconnects the 6-voltbattery B1 from all of the logic circuits.

2. Off-Hook Condition

When the telephone comes off hook, the hook switch S1 closes and placesthe negative voltage at negative terminal of bridge CR11 to resistorR37, causing diode CR3 to be forward-biased. This results in currentflowing between the base and emitter of transistor Q4 and through diodeCR7, and thus through speech hybrid circuit U11 to the positive terminalof the bridge CR11. Resistor R37 has an impedance on the order of 10,000ohms, and therefore only a small amount of current is being drawn fromthe central office loop current, and is not sufficient to cause anoff-hook state to be sensed at the central office. However, transistorQ4 is now forward-biased, causing the collector of transistor Q4 to pullpositive and in turn forward-bias transistor Q5. Transistor Q5 nowbecomes heavily conducting, which bypasses the current around the hookswitch S1. Since transistor Q5 is conducting, the collector of thattransistor is low, which is a parallel current path around the hookswitch S1 and therefore maintains transistor Q4 in an ON condition. Theconfiguration of transistor Q4 and Q5 are such that they latch eachother ON once they are in conduction, so long as there is DC currentthrough the circuit. Thus, the combination of transistors Q4 and Q5function in a manner similar to a semiconductor controlled rectifier(SCR) except that transistor Q3 serves as a means to turn transistors Q4and Q5 off, thus interrupting the loop current between the tip and ringconductors, except as is discussed below. Normally, loop current willcontinue to flow with transistors Q4 and Q5 ON, even if the hook switchS1 is open, i.e. in the on-hook condition.

Resistors R37, R36, R26 and R35 form a voltage divider network thatmonitors the charge across super capacitor C4. This is a safety featurethat keeps the microprocessor U4 and other logic circuits from operatinguntil there is a sufficient operating voltage provided for by the loopcurrent. Resistor R26 is a positive temperature resistor thattemperature compensates in the voltage divider circuit, in order toallow correct turn-on of the logic circuits under a wide temperaturerange. The operation of this turn-on circuit is as follows: after thehook switch S1 is closed and loop current flows between the tip and ringconductors and through transistors Q4 and Q5, the junction of thecathode of diode CR3 and resistor R37 are held negative, and supercapacitor C4 then charges to within one diode drop (0.7 volts due todiode CR10) of the Zener voltage of diode CR7. When super capacitor C4develops sufficient operating voltage (on the order of 4.5 to 5.5volts), the emitter-base junction of transistor Q2 becomesforward-biased. After this occurs, the collector of transistor Q2 pullsto about 5 volts, causing transistor Q12 to conduct; because thistransistor is configured as an emitter follower, the emitter of thattransistor approaches the 5 -volt potential, causing capacitor C21 tocharge through resistor R86. Once capacitor C21 charges to a 5-voltlevel, a logic "one" condition is presented to the gate of integratedcircuit U12. The output of that gate terminal of circuit U12 drives thegate of field effect transistor Q11, causing Q11 to turn on. When inconduction, field effect transistor Q11 turns on the switched logicground connected to the drain of that device. All logic circuits are nowin a powered up state and ready to perform their tasks. The first dutyof the microprocessor U4 following the off-hook condition and thepowering up from loop current as discussed above, is to place relaysK102 and K101 in the reset, or deenergized condition. The drawing ofFIG. 1 shows these relays in that deenergized state.

All of the loop current from the central office passing through the tipand ring conductors passes through the 5-volt power supply of thetelephone, and will also pass through the hybrid circuit U11, since thatcircuit is in series with the current path. Hybrid circuit U11 is aconventional device, such as a Texas Instrument TMS 1705 or SamsungKA2412, and converts the telephone tip and ring conductors from an audiotwo-wire circuit to a four-wire circuit, wherein transmitter power ofthe handset associated with the telephone is separated from the receiverpower. To this end, transistor Q14 serves as an electronic switch thatis capable of turning the handset transmitter either OFF or ON undercontrol from the microprocessor U4. Transistor Q8 is also an electronicswitch which is capable of turning the receiver either OFF or ON undercontrol of the microprocessor U4. Any audio modulation associated withthe loop current from the central office is detected by hybrid circuitU11, is amplified and is then sent to the receiver of the handset viatransistor Q8. When the transmitter picks up audio from the handsettransmitter and transistor Q14 is ON, hybrid circuit U11 amplifies thatsignal and modulates the loop current with the received audio signalfrom the handset. Hybrid circuit U11 is a bridge circuit that preventsthe audio from being fed back to the receiver of the handset.

The microprocessor U4, which suitably is a 655 C 150 manufactured by GTEMicrocircuits, is also capable of generating audio frequencies between10 Hertz and 3000 Hertz. All modem tones, all DTMF digits, all cointones, busy signals and the like associated with the conventionaloperation of a telephone receiver and transmitter are generated at theanalog output of the microprocessor U4 in accordance with standardoperating conditions, and inserted into the transmitter input of thehybrid circuit U11. Hybrid circuit U11 then amplifies the audio signalsfrom the microprocessor U4 and presents those signals to the telephonetip and ring conductors for transmission to the central office.

The DC current through the hybrid circuit U11 results in a small DCvoltage dropped across that circuit, causing the emitter-base junctionof transistor Q7 to be forward-biased. When transistor Q7 is insaturation, the collector is pulled positive, causing diode CR5 to beforward-biased. Diode CR5 in turn initiates conduction in transistor Q6,which in turn applies the battery B1 across the logic supply of thetelephone. Unless the 130-volt inverter (note the extreme left-handportion of FIG. 1A) is ON, the battery will not be required to supplyany power to the circuit. In fact, the excess current through the loopwill be diverted into the battery, resulting in a positive charge beingapplied to the electronics as long as the phone is off-hook. Thus, itwill be seen that the power for of the electronic circuitry is entirelyderived from the direct current component of the loop current along thetip and ring conductors.

3. Audio Receiver

The telephone shown in the drawing has the ability to recognize andidentify conventional audio signals being presented to the telephonefrom the central office along the tip and ring conductors, and torespond accordingly. Typically, the tones required to be identified are:380 Hertz for call processor answer; 400 and 440 Hertz for OCC answertone; bell bong tone of 941/1477 Hertz, followed by decaying dial tone;and 300 Baud FSK signals. The audio take-off point is the positiveterminal of bridge CR11 and is presented to integrated circuit U15Awhich is a two-pole high-pass filter. The purpose of the filter U15A isto remove all traces of 60 Hertz power line distortion so as to avoidjamming the receiver. The output of the high-pass filter U15A goesthrough buffer U15B and to the notch filter U15D. Unless the bong toneis being detected, the notch filter U15D is turned off by themicroprocessor U4. In that OFF state, the notch of the filter U15Dbecomes an "all pass" filter. Thus, the output of filter U15D ispresented to the squaring circuit of U15C. Resistor R114 and capacitorC28 develop the power for the operational amplifier U15. Themicroprocessor U4 analyzes zero crossings of the audio signal todetermine the frequency of the incoming signal and its pulse duration.When the bong tone is being received, the microprocess U4 inserts thenotch filter U15D in the circuit to trap out the 1477 Hertz element ofthe 941/1477 Hertz combination. The microprocessor then looks for the941 Herts element of the tone pair.

4. Open Loop Coin Check

The telephone has associated conventional coin switches which are usedto monitor the coins that are deposited. When the telephone isinstalled, the base rate required for a local call is programmed intothe memory of the EPROM U3; this is a memory device that is non-volatileand is field-programmable. When a local call is dialed from thetelephone, the central office will look at the telephone to determine ifthe local base rate has been deposited. A preferred method foraccomplishing this is through the use of an open loop coin check. Thisoccurs when the central office removes the battery voltage from thetelephone line for several milliseconds, and then applies a highimpedance, high voltage battery to the tip conductor with the ringconductor open. Again, this is a conventional technique used inpresent-day coin telephone systems.

The telephone shown in the drawing detects this loss of loop current bymonitoring normal and reverse current detectors U9 and U10. When theloop current is interrupted, the telephone responds by comparing theamount of coinage deposited with the base rate programmed in the memoryof the EPROM U3. If the base rate has been satisfied, the telephoneenergizes relay K102 by activating its set contacts. Any coin depositedin the telephone will cause the associated coin present switch to closeand stay closed as long as the coin is held in escrow. Only theapplication of a negative or positive 130-volt refund or collect pulsewill remove the coin from escrow. Once the loop current from the centraloffice stops, transistor Q7 turns off, removing the forward bias fromdiode CR5. However, capacitor C7 is charged at this time and maintainsfield effect transistor Q6 in the ON state until capacitor C7 dischargesthrough resistor R42. This discharge time is on the order of 7 to 8seconds. Consequently, the battery B1 remains connected to the logicpower supply for this time interval. The forward bias on transistors Q2and Q12 is also lost when loop current is interrupted. However, thecharge on capacitor C21 maintains a logic "one" state on gate U12 for 4to 5 seconds after transistor Q12 turns off. The discharge timingassociated with this portion of the telephone circuitry is determined byresistor R87. As long as capacitor C21 is charged, the logic switchedground is applied to the microprocessor U4, which will continue tooperate. Several milliseconds after the removal of loop current, thecentral office will place a negative 48-volt potential on the tipconductor through a high impedance ground detector, which is typicallyon the order of 8000 ohms. If the base rate is accurate, current flowsfrom the negative 48-volt battery terminal in the central office,through the central office ground detector circuit and to the tipconductor of the telephone line. From the tip conductor of thetelephone, the current flows through the energized contact of relay K102to the deenergized contact of relay K101. The signal is then presentedto the coin present switch, then to the coil of the escrow relay ERCback to the telephone and through the couplers U100 and U101, and thenceto the earth ground of the telephone. Since the positive terminal of thecentral office 48-volt battery connects to earth ground, then there iscurrent flow through the coin detector circuit to the central office onthe order of 7 milliamperes or less. This satisfies the coin test andthe central office will then let the call proceed. Since about 40milliamperes or greater of current flow through the escrow relay ERC isrequired to refund or collect a deposit, the monies held in escrow willremain in escrow. Further, since about 30 milliamperes of current arerequired to activate the couplers U100 and U101, no output will be givenfrom these circuits at this time. The central office then removes the48-volt current and reconnects normal central office battery. Themicroprocessor U4 detects this reapplication of the "talk" battery viacircuits U9 and U10. The microprocessor U4 then resets the relay K101.This removes the earth ground connection to the tip conductor via theescrow relay ERC.

If the base rate has not been satisfied, then the telephone will notoperate relay K101 and therefore there is no current flow into the tipconductor to activate the coin detector. The central office willtherefore block a call that has not satisfied the base rate requirement.

5. Coin-Free Calls (Fake Coin Ground)

If appropriately programmed into the telephone, a local call may beallowed to be completed without the deposit of any coins. Assuming thatthe telephone is so programmed, both relays K101 and K102 will beenergized when the coin check is made. As a result, the coin detectcurrent will pass through the tip conductor of the telephone and throughthe energized contact of relay K102 as described above. However, thesensed current will be applied to the bridge CR111 via the energizedcontact of relay K101. This sensed current will pass through the bridgeCR111, through transistor Q109, resistor R132 and back to the bridgecircuit CR111 to earth ground. This current flow criteria satisfies thecoin check and the central office processes the call as if a coin hadbeen deposited. The bridge CR111, transistor Q109, resistor R133,capacitor C106 and resistor R132 act as an inductor simulator having arelatively low DC resistance on the order of 1,000 ohms and a relativelyhigh AC impedance on the order of 100,000 ohms, and uniquely allows thesimulated coin ground to be applied during the talk state continuously,especially for continuous ground type installations for coin ground.

6. Refunds or Collections from Central Office

Typically, the central office initiates a refund or collect signal bydisconnecting the central office "talk" battery, which in turninterrupts loop current along the tip and ring conductors. The telephonedetects this state by monitoring output from circuits U9 and U10. If acoin is in the telephone, the relay K102 is energized. Severalmilliseconds later, the central office will apply either a negative130-volt (refund) signal or a positive 130-volt (collect) pulse to thetip conductor with the ring conductor either open or short-circuited tothe tip conductor. This high voltage pulse will enter the tip conductorof the telephone and pass through the energized contact of relay K102,through deenergized contact of relay K101, through the closed contact ofthe coin present switch, through the escrow relay coil ERC and throughopto-couplers U100 and U101 to earth ground. Since the positive andnegative 130-volt supplies in the central office are referenced to earthground, a heavy current greater than 45 milliamperes will flow throughthe escrow relay ERC when these voltages are applied during the refundor collect cycle. This current will cause the escrow relay to eitherrefund or collect the monies held in escrow, dependent on the directionof current flow. A positive current through the ERC relay causes moniesto be collected, and if the negative 130-volt supply is applied, thecurrent will flow in an opposite direction through the ERC relayresulting in a refunding of monies held in escrow. Currents that aregreater than 30 milliamperes through the ERC relay will operated eithercoupler U100 or U101. A refund pulse will activate U100, causing thecollector of U100 to pull to a logic "one" (5 volts). This signals themicroprocessor U4 that the money has been refunded. A collect pulseactivates coupler U101, causing the collector of U101 to pull to logic"zero," which is inverted to a logic "one" by circuit U103C, which turnson transistor Q102 and sets latch U103A/B. The microprocessor U4monitors the collector of transistor Q102 to determine if the collectpulse is present. Latch U103A/B is for the purpose of monitoring collectpulses if the phone is on-hook, the microprocessor is shut down and thecollect pulse is applied. This will be described below in greater detailwith reference to the "hang up" circuit description.

As soon as a coin clears the hopper in the telephone, the coin presentswitch opens in a conventional manner. The central office monitors thecurrent flow to the telephone during the collect or refund cycle, and apulse is applied for approximately 750 milliseconds. If, at the end ofthis period, current is still present to the escrow relay ERC, thecentral office interrupts and treats this as a "stuck" coin condition.Several more collect or refund pulses will be applied, and if the coinpresent switch does not open, the central office sounds an alarmindicating the telephone is out of service due to a stuck coin. Afterremoving the collect or refund pulse, the central office reapplies the"talk" battery. The telephone then detects this condition by monitoringcircuits U9 and U10 for the reapplication of loop current. When thishappens, the telephone resets relay K102. Note that at any time relayK102 is energized, the 130-volt inverter is disconnected from thecircuits of the telephone. This prevents the central office minus orpositive 130-volt signal from dumping into the inverter circuit.

7. Refunds or Collections from Telephone

The microprocessor U4 also has the ability to control the collection orrefunding of a deposited coin. Under these circumstances, themicroprocessor U4 generates a high frequency square wave and presents itto the 130-volt inverter. The output of the inverter passes through thedeenergized contact of relay K102, the other set of contacts for relayK102 being open, therefore keeping the inverter voltage from feedingonto the tip and ring conductors. Inverter current, after passingthrough relay K102, passes through the coin present switch to the coilof the escrow relay ERC to detectors U100 and U101 and thence to earthground. This activates the escrow relay ERC and coins are eitherrefunded or collected, depending upon the polarity of the invertersignal. The microprocessor U4 monitors couplers U100 and U101 todetermine if the coins become stuck, or when they are cleared. Toconserve energy, the microprocessor U4 applies a short pulse to therelay and if the coin activity stops, the pulse is not repeated.

8. Hang Up Circuit Condition

The microprocessor U4 determines if the telephone is in on-hook oroff-hook conditions by monitoring the voltage at the junction ofresistors R37 and R36. The on-hook condition causes this junction to goto the logic "one" or 5-volt potential. Under these conditions, loopcurrent continues to flow heavily into the circuit, since transistors Q4and Q5 are still conducting. Upon hang up of the hook switch,transistors Q2 and Q12 are turned off, and therefore capacitor C21begins to discharge through resistor R87. The circuit elements areselected such that the discharge time is approximately 5 seconds. Themicroprocessor U4 does all processing chores necessary to terminate thecall before opening the circuit connection between the tip and ringconductors, to effectuate an "on-hook" condition to appear to thecentral office. This permits a significant amount of loop current to beavailable to accomplish all tasks that require large amounts of powerbefore the disconnect is complete. These tasks performed by themicroprocessor U4 after hanging up and resuming the on-hook condition,but before terminating loop current, constitute various call accountingfunctions (such as coin status, for example) which may be stored intothe E2ROM to be preserved for future reference. The microprocessor U4also resets the latch circuit U103A/B at this time and energizes relayK102. The microprocessor then places a logic "zero" on the base oftransistor Q3, causing that transistor to conduct. With transistor Q3 inconduction, the emitter-base junction of transistor Q4 becomes starvedfor current and therefore that transistor shuts off causing transistorQ5 to be shut off. This opens the central office loop current pathbetween the tip and ring conductors. The central office will considerthe call to be abandoned after 1.5 seconds without the flow of loopcurrent. The microprocessor will maintain Q3 in the ON state for thefull 1.5 second period, even if hook switch S1 is reclosed during thisinterval. This keeps the user of the instrument from interferring withthe disconnect cycle, and also prevents the use of a technique known ashook switch fraud.

After the central office recognizes the on-hook condition, the ringconductor is open and the central office then places a negative orpositive 130-volt pulse onto the tip conductor to clear any coins thatmay be in the telephone, as was described above. After an interval of 5seconds, the line opens and loop current to the telephone is interruptedand transistor Q11 turns off the microprocessor power, as well as mostlogic power. The battery B1 is also disconnected 8 to 10 seconds afterloop current interruption and capacitor C7 discharges and turns thefield effect transistor Q6 off. Diode CR22 is a safety device to ensurethat the telephone is on-hook and that the microprocessor U4 operatedproperly and opened the telephone line. When switched logic power isturned off, a negative pulse is differentiated through capacitor C40 todiode CR22, which forward-biases transistor Q3, which turns offtransistors Q4 and Q5 in the event that they are still in conduction atthis time. Note that the logic may be run from a combination of thepower from super capacitor C4 and the battery B1 in the absence of anyloop current; however, the telephone is capable of operating its logicsolely from the charge in capacitor C4 for the 5-second interval afterhang-up. The battery B1 is only necessary to provide the high energypositive or negative 130-volt pulse to the escrow relay ERC, asdiscussed above.

9. Telephone Self-Originate and Answer Functions

The telephone has the ability to originate an outgoing call, which mayfor example be to a computer via a modem terminal. The telephone has theability to also receive and transmit 300 Baud FSK signals to communicatewith computers with modems for the purpose of reporting phone status,change programming options, call accounting, coin accounting and thelike.

To answer an incoming call, the ring signal from the central office isdetected by circuit U8. This device sets the latch U104B/C. When thering signal stops, latch U104A output goes high, which undergoes a levelshift and the hook shift S1 is bridged via transistor Q106. Theremaining off-hook process is the same as described above. ResistorsR122 and R119, diode CR115 and capacitor C110 serve the function ofscreening types of incoming signals so that normal incoming signals willnot activate the telephone to the self-answer condition.

If the telephone is to originate a call, the microprocessor U4 sets thelatch U104B/C on the previous call so that when it completes a call andthe line is disconnected, transistor Q106 turns on and once againbridges the hook switch S1.

It will be appreciated that the telephone of the present inventionprovides a means for powering the telephone entirely from the DCcomponent of loop current along the tip and ring conductors during theoff-hook condition, and further includes a charge storage element andmeans for delaying the powering of the telephone until the energy withinthe charged storage element (super capacitor C4) reaches a predeterminedlevel to ensure that adequate voltage is available for the logiccircuitry.

The telephone described above provides a system in which loop currentflowing from the central office and through the tip and ring conductorsmay be maintained after resuming the on-hook condition for apredetermined period of time to accomplish certain telephone functions,such as call accounting or to frustrate attempts at hook switch fraud.This telephone also provides a means for utilizing loop current to powerthe telephone to initiate outgoing calls after resuming the on-hookcondition in order to report various call accounting functions such asthe amount of money in the coin box, a coin box full condition, toreport a phone abnormality or to report a fraud or theft attempt.Further, the use of a simulated inductor in this system facilitates theplacing of coin-free calls as programmed into the telephone, whileavoiding the difficulties associated with the use of a resistive onlyelement in the circuit.

This telephone system further provides means including a single coinground relay (K102) for providing an open-loop coin check, and means atthe local telephone for sensing coin conditions and operating the escrowrelay without control from the central office. Further, after loopcurrent has been interrupted and the telephone electronics have beendeenergized, coin totalization can be accomplished following receipt ofa collect pulse from the central office by the resetting of latchU103A/B before shut-down.

What is claimed is:
 1. A pay telephone for use in a telephone systemwith a central office having a coin trunk including tip and ringconductors coupled to said telephone and in which loop current flowsfrom said central office and through said telephone responsive to anoff-hook condition at said telephone being sensed by said centraloffice, said pay telephone comprising:a hook switch for closing acircuit path in said telephone between said tip and ring conductors sothat said loop current flows through said telephone; and means forestablishing said loop current flow through said pay telephone for apredetermined period of time after said hook switch is opened.
 2. Thepay telephone system recited in claim 1 wherein said telephone comprisesescrow relay means and said loop current establishing means comprisesmeans for continuing loop current flow for a period of time sufficientto operate said escrow relay means.
 3. The pay telephone system recitedin claim 1 wherein said establishing means comprises means forcontinuing loop current flow through said telephone for a period of timeto prevent any attempt at hook switch fraud through rapid keying of saidhook switch.
 4. The pay telephone system recited in claim 1 furthercomprising means for continuing loop current flow through said telephonefor a period of time sufficient for preventing said central office fromdisconnecting said tip and ring conductors until an incoming or outgoingcall is legitimately terminated.
 5. The pay telephone system recited inclaim 1 wherein said telephone comprises means for providing a callaccounting function for each completed call, said establishing meanscomprising means for continuing said loop current flow through saidtelephone for a period of time sufficient to complete said callaccounting function.